1. Field of the Invention
The present invention relates to a semiconductor memory device, and more specifically to a semiconductor memory device having a strobe buffer and an output buffer.
2. Description of the Background Art
In a synchronous SRAM (Static Random Access Memory), a data signal is read from a memory cell to an output buffer in synchronism with the rise of a clock signal, and further is output from the output buffer to an external MPU (Main Processing Unit). Further, a timing signal referred to as a strobe signal is output from a strobe buffer to the external MPU in synchronism with the rise of an inverted signal of the clock signal. The logic level of the strobe signal changes from "H" (logic high) to "L" (logic low) to "H" to "L" and so on in synchronism with the rise of the inverted signal of the clock signal. The MPU takes in the data signal from the output buffer when the level of the strobe signal changes.
Normally, the time that passes from the rise of the clock signal to the time when the data signal is output by the output buffer is designed to equal the time that passes from the rise of the inverted signal of the clock signal to the time when the level of the strobe signal changes. Therefore, there is a time gap between the time when the level of the strobe signal changes and the time when the data signal output from the output buffer changes, the time gap being equal to the clock pulse width. As a result, the data signal is taken in by the MPU when the signal is most stable.
Normally, the data signal output from a synchronous SRAM contains multiple bits, causing a plurality of output buffers to operate at the same time. A large amount of electrical current flows at this time, and the electrical potentials of the power supply line and the ground line of the output buffers become unstable.
If a power supply line of an output buffer and a power supply line of a strobe buffer are connected, the instability of the electrical potential of the power supply line of the output buffer affects the electrical potential of the power supply line of the strobe buffer to become unstable as well. Moreover, if a ground line of an output buffer and a ground line of a strobe buffer are connected, the instability of the electrical potential of the ground line of the output buffer affects the electrical potential of the ground line of the strobe buffer to become unstable as well.
As a result, when a noise pulse appears on the strobe signal or when the level of the strobe signal changes before the noise pulse disappears, it becomes impossible to acknowledge the level changes of the strobe signal accurately, which leads to the problem of the MPU taking in the data signal at an inappropriate time.